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Description
Geologic mapping within the northern Sacramento Mountains delineated four mid-Tertiary sequences separated by three angular unconformities. The oldest sequence, Ts1, consists of three intervals: 1) A basal coarse grained lithic arkose derived primarily from Precambrian rapakivi granite. Deposition of this interval occurred prior to mid-Tertiary detachment faulting. 2) Well-rounded boulder conglomerates and sedimentary breccia derived from upper-plate Precambrian rapakivi granite, interbedded with lacustrine and playa deposits. Deposition of this interval occurred during the first phase of detachment faulting. 3) Variably welded dacite/rhyolite ashflow tuffs and minor basalt lava flows. Deposition of this interval occurred during the latter part of the first phase of extension. The second sequence, Ts2, consists of interbedded freshwater limestone which is often silicified; and alluvial sediments, usually of mid- to outer-fan facies. Ts2 sediments, which usually dip 15 - 40 degrees to the southwest, overlie Ts1, which usually dip 30 - 70 degrees to the southwest, along an angular unconformity. The third sequence, Tvr, consists of an upper portion of several ashflow tuff cooling units, indicated by varying color, phenocrystslithics, and degree of welding. Tvr, which usually dips 10 - 15 degrees to the southwest, unconformably overlies Ts2 strata. The youngest mid-Tertiary sequence, Ts3, consists of alluvial sediments that overlie unconformably Ts1, Ts2, and crystalline rocks. Ts1 and Ts2 are in fault contact with chloritized, well-broken to cataclastic gneisses and granites. This low-angle normal detachment fault is part of the regional Mojave-Sonoran extensional terrane. The Ts1/Ts2/Tvr hanging wall extended brittlely along high angle planar and listric normal faults, which rotated strata. The three angular unconformities between Ts1/Ts2, Ts2/Tvr, and Ts3/Ts1-Tvr indicate three phases of extension occurred. Syndepositional deformation of both Ts1 and Ts2 produced growth faulting geometries where older Ts1 sediments dip more strongly to the southwest than the younger Ts2 sediments.Tilted volcanic rocks within the basal portion of Tvr yielded K/Ar ages ranging from 18.6 to 14 Ma. Flat lying basalts yielded a K/Ar age of 14.6 Ma. Therefore, the main phases of detachment faulting occurred prior to 14.6 Ma. Basin and Range style normal faulting chopped up the upper- and lower-plate rocks, as indicated by minor dip-slip separation of the 14 Ma flat lying basalt. Apparently, rotational deformation related to detachment faulting ceased around 14 Ma within the Sacramento Mountains. Mineralization accompanied faulting. Barite, specularite, CuOx, quartz, and MnOx grew along lower-plate lense faults and northeast-dipping rotation faults that cut upper-plate rocks. Anomalous concentrations of silver, copper, barium, antimony and arsenic define alteration halos around many of these faults, indicating a paleohydrothermal system similar to detachment fault hosted precious metal deposits, such as Mesquite and Picacho mines. These alteration halos define favorable sites for drilling, in order to locate buried economic grade precious metal deposits.
